I examine treatment eﬀect heterogeneity within an experiment to inform external validity. The local average treatment eﬀect (LATE) gives an average treatment eﬀect for compliers. I bound and estimate average treatment eﬀects for always takers and never takers by extending marginal treatment eﬀect methods. I use these methods to separate selection from treatment eﬀect heterogeneity, generalizing the comparison of OLS to LATE. Applying these methods to the Oregon Health Insurance Experiment, I ﬁnd that the treatment eﬀect of insurance on emergency room utilization decreases from always takers to compliers to never takers. Previous utilization explains a large share of the treatment eﬀect heterogeneity. Extrapolations show that other expansions could increase or decrease utilization.